1. Field of the Invention
The present invention relates to a process for separating two or more components of a plastic material for the purposes of recycling, with particular application to purifying poly(ethylene terephthalate) (PET) during recycling procedures.
2. Background of the Related Art
Recycling of waste products has become increasingly common in the last couple of decades, and the recycling of plastics is one of the most important and widely carried out with many industries and households around the world actively involved.
A multitude of everyday consumer items are made from plastics, such as bottles, bags and product packaging. Drinks bottles, for example, contain a number of different polymers. Separation of a post-consumer plastic waste stream into its unique constituent polymers so they can be subsequently re-used is the most difficult and expensive step of the recycling process.
For example, one such polymer which forms part of the plastics is poly(ethylene terephthalate), also known as PET. PET is the biggest volume post consumer plastic to be recycled worldwide with many plants in Europe and the USA being involved in its recycling, and with more plants being under construction or being planned in other parts of the world.
One vital aspect of PET recycling is the removal of other polymers such as poly(vinyl chloride) (PVC), polystyrene (PS), acrylonitrile butadiene styrene (ABS), or poly(lactic acid) (PLA), from the PET feed stream. However, a problem associated with this is the fact that the e.g. PVC degrades at PET extrusion temperatures. This results in a reduction of PET's intrinsic viscosity and causes discoloration of the PET. The removal of PVC is also difficult due to the similar densities of the two polymers.
One separation technique which has been employed uses spectroscopic sorters; however, this technology only has a limited level of accuracy in separating PET and PVC.
Another technique takes advantage of the relative properties of PVC and PET at elevated temperatures. PVC softens and becomes tacky when heated to about 200° C., whereas PET remains rigid up to about 260° C. The technique involves a stainless steel belt, onto which flakes of the plastic to be separated and purified are placed. The belt is heated up to about 200° C., and the PVC becomes tacky and sticks to the belt, separating the PVC impurity from the PET, which remains rigid.
However, this technique also has significant disadvantages. To remove the PVC from the steel belt, the belt then has to be continuously scraped or dismantled and the PVC which is stuck to the steel has to be cleaned off before the belt can be used again for a subsequent separation process. This is time consuming, costly, inefficient and inconvenient.
It would therefore be desirable to devise a process and an apparatus for the separation of constituents of a plastic material for plastic recycling which obviates the disadvantages of the existing processes detailed above.